Gliobastoma multiforme (GBM) is the most common primary brain tumor and is incurable, with an average survival of 12-18 months. Numerous studies have clearly established that the cell cycle kinases cdk4 and cdk6 are activated during the pathogenesis of GBM. The most common mechanism for activation of cdk4/6 in GBM is homozygous deletion of the INK4 locus (containing the p16INK4a and p15INK4b genes). Less common mechanisms include amplification of the cdk4, cdk6, and cyclin genes themselves, and homozygous deletion of the p18INK4c gene. Since cdk4 and cdk6 are activated by these mechanisms in ~90% of GBMs and GBM has been clearly shown to be addicted to activated cdk4/6, these proteins represent an extremely promising molecular target for the treatment of GBM. Recently, a collaborative effort between David James' lab (UCSF Cancer Center) and Todd Waldman's lab (Lombardi Cancer Center, Georgetown Medical School) has demonstrated that PD0332991, a specific pharmacological inhibitor of cdk4/6, is remarkably effective in halting the growth of GBM in preclinical models (Cancer Res 70:3228-38, 2010). This study motivated the first clinical trial for testing a cdk4/6-specific inhibitor in GBM. Accrual for this UCSF-base trial was initiated in September, 2010, and 17 patients with treatment-refractory, recurrent GBM have thus far been enrolled. This grant application is designed to enable high quality basic science and translational research focusing on the utility of cdk4/6 inhibition as a therapeutic target for GBM. The proposal has three aims. In Aim #1, we will distinguish between the roles of cdk4 and cdk6 in GBM pathogenesis and determine which enzyme is the key target of inhibition by PD0332991 in GBM. In Aim #2, we will determine the mechanisms of intrinsic and acquired resistance to cdk4/6 inhibition in GBM. In Aim #3, we will evaluate the efficacy and toxicity of PD0332991 in combination with radiotherapy and temozolomide.